There is currently specified a procedure known as SSDT (Site Selection Diversity Transmit power control) for use in a receiver of a WCDMA (Wideband Code Division Multiple Access) based system. The SSDT is a macro diversity method used in the soft handover mode, where the UE selects one of the cells from its active set of cells to be the primary cell. All other cells in the active set are then classified as being non-primary cells. The main objective is to transmit on the downlink (base station (BS) to UE) only from the primary cell, thus reducing the interference caused by multiple transmissions from other BSs in the soft handover mode. During conventional soft handover the UE can be simultaneously receiving transmissions from more than one BS.
In order to select a primary cell, each cell is assigned a temporary identification (ID), and the UE periodically notifies connecting cells of the primary cell's ID. The non-primary cells selected by the UE switch off their transmission power.
In the SSDT technique, as described above, the UE signals a desired Node B's Cell ID on the uplink using the DPCCH (Dedicated Physical Control Channel). In practical deployment scenarios, however, there exists some non-zero probability for the occurrence of an error in the FBI (Feedback Information) bits that are used to transmit the Cell ID. Because of this non-zero error probability it can occur that the Node B signaled as the primary cell by the UE detects the code word erroneously, and thus does not assume the role of the primary cell for the signaling UE. In this a case the Node B does not transmit the downlink DPCH (Dedicated Physical Channel), as it interprets itself as being a secondary cell. As there is no downlink signalling to inform the UE as to which cell(s) have interpreted their role as being either the primary or the secondary cell, a UE functioning according to the current specification will attempt to receive the DPCH from the cell that the UE assumes to be the primary cell. However, a resulting attempt to decode non-existent bits (noise) on the downlink DPCH leads to an undesirable degradation in decoder performance.